1. Field of the Invention
The present invention relates to the vacuum sizing of extruded bodies and, more particularly, to a vacuum sizing apparatus having a sizing surface defined by a clamped-together assembly of structural segments with vacuum channels formed as grooves in side walls of the segments.
2. Prior Art
In the formation of continuous lengths of thermoplastics material, it is customary to utilize an extruder having a die through which heated material is extruded. The die constricts the flow of plastics material, causing it to conform to a desired cross-sectional shape and size as it exudes and issues from the die.
Since some thermoplastics material tends to expand upon leaving the die, and since the plastics material emerging from the die is in a heated, deformable state, it is generally the practice to feed the extruded material to a sizing fixture which causes the material to maintain or assume the desired cross-sectional configuration and size as the material cools and rigidifies.
One type of sizing fixture which has been used with good success provides an elongate sizing surface positioned downstream from the extruder along the path followed by the body of plastics material as it emerges from the die. A large number of vacuum channels, in the form of small diameter holes, open through the sizing surface. A system of conduits is provided which communicates the holes with a vacuum source. Sizing fixtures of this type are known in the art as "vacuum sizing apparatus" due to their use of the principle of vacuum evacuation across the sizing surface to maintain outer wall portions of the plastics body in conforming engagement with the sizing surface as the body moves along the sizing surface.
One type of vacuum sizing apparatus employs a plurality of sizing units arranged in sequence along the path of travel followed by the extruded thermoplastics material. Where this approach is utilized, each of the sizing units has a sizing surface which operates on only a portion of the outer wall surface of the plastics body. A system of this type is described in U.S. Pat. No. 4,020,136.
Another type of vacuum sizing apparatus has a sizing surface which defines an elongate passage having the desired cross section unto which outer walls surfaces of the extruded plastics body are to be conformed. As the body travels through the passage, its outer walls are drawn into conforming engagement with the sizing surface. In some instances, a vacuum sizing apparatus of this type is positioned in a fluid-containing vacuum evacuated cooling chamber, as described in U.S. Pat. No. 3,804,574. In other instances, the vacuum sizing apparatus is provided with an encompassing cooling system, as described in U.S. Pat. No. 3,668,288. In still other applications, the vacuum sizing apparatus is located upstream and/or downstream from one or more cooling units, as described in U.S. Pat. Nos. 3,296,661 and 3,229,005.
In order for vacuum sizing apparatus to function properly in certain applications, it is important that the vacuum channels which open through the sizing surface be of very small cross-sectional area. Holes having a diameter of about 0.001 inch, and a corresponding cross-sectional area of about 0.0000008 square inches, are found to be quite suitable for use in many vacuum sizing operations. A large number of these holes are needed and they are ordinarily uniformly spaced across the vacuum sizing surface.
In fabricating vacuum sizing apparatus, it has long been the accepted practice to form the needed vacuum channels by individually drilling a multitude of small diameter holes. Drilling a large number of minute holes at accurately spaced intervals across a large sizing surface is time consuming and expensive. In fact, the most costly aspect of manufacturing most vacuum sizing units relates to the steps involved in accurately positioning and drilling the many tiny holes needed to form vacuum evacutation channels.
In many instances, it is desirable to form the structure which defines a vacuum sizing surface from a relatively hard, corrosion resistant material such as stainless steel. As is well known to those skilled in the art, hard materials such as stainless steel are difficult to drill, particularly where the size of the holes to be drilled is quite small. Accordingly, it has become customary to accept sizing apparatus fabricated from less desirable materials which are more adaptable to drilling, or to pay very high costs for the fabrication of vacuum sizing structures from higher quality materials that are difficult to drill.
Still another problem with previously proposed vacuum sizing apparatus is that the small diameter vacuum holes tend to become clogged during use and are difficult to clean. In some instances, the holes become so clogged as to practically defy cleaning. In most instances, it is difficult to effect a thorough cleaning of the holes.